1. Field of Invention
The present invention relates to an accumulation time correcting device for a photoelectric converter composed of self-scanning charge-accumulation elements capable of automatic gain control on the basis of the average of the incident light.
2. Description of the Prior Art
A self-scanning charge-accumulation device such as a charge-coupled device (CCD) is often employed as a photosensor, for example, in an automatic focusing unit for camera. In such device, the charges are accumulated for a predetermined time, and the signals are transferred when the charges are accumulated up to a predetermined level suitable for signal processing. In such a device, an excessively long accumulation time leads to the charge overflow phenomenon, thus destroying the proportional relationship between the amount of light received and the amount of accumulated charge, while an excessively short accumulating time gives rise to an insufficient S/N ratio since the signals are mingled with noises such as dark current. In the use of such self-scanning charge-accumulation devices for an automatic focusing unit, therefore, it is desirable to regulate the charge accumulation time according to the brightness of the object, so as to maintain the accumulated charge at an optimum level.
In order to obtain an output signal of an appropriate level from the self-scanning charge-accumulation device, it has conventionally been proposed to monitor the output of said device obtained at a predetermined accumulation time, to regulate said accumulation time on the basis of said output, and to repeat the above-described procedure until the output of said device reaches a desired level, prior to the output signal processing.
In such a method, the accumulation time of the self-scanning charge-accumulation device is not determined on the basis of real-time but is predicted in response to the preceding accumulation time maintained in memory and to the output of the device.
In the case of an object illuminated with a light source intermittently activated at the line supply frequency such as a fluorescent lamp, the start of charge accumulation has to be synchronized with the on-off action of the light source in order to eliminate the influence of the power supply frequency. Thus, in order to achieve synchronization with both the widely employed frequencies of 50 and 60 Hz, the charge accumulation should be started every 100 milliseconds, or at a frequency of 10 Hz which is the greatest common measure of the abovementioned two frequencies. In such a method, however, the data can only be obtained every 100 milliseconds even when a short accumulating time is required, and a prolonged data processing time will be required if such photoelectric converter is employed for automatic focus control.
In order to avoid synchronization for the frequencies of 50 and 60 Hz, it is also proposed to provide an auxiliary photoelectric sensor for measuring the average brightness of the object in addition to the main photoelectric sensor employed for signal processing, and to determine the accumulation time on real-time basis from the output of said auxiliary sensor. Though this method avoids the necessity for the above-described synchronization, the average of the accumulated charge may be significantly different, if the main sensor is composed of a linear sensor of plural bits, from the peak charge accumulation in a particular bit depending on the distribution and contrast of the brightness of the object. More specifically, in the above-described method in which the auxiliary sensor measures the average luminance of the object and the charge accumulation of the main sensor is terminated when said average reaches a determined value, the obtained signals may not conform to the brightness pattern of the object since the accumulated charge may be saturated already in certain bits of the main sensor.